Add in a resolver task. The resolver takes an extension and a set of operations that can be used to get library coresponding to specified extension. The task will run through each operation until required extension is found.
ant/core/trunk/src/main/org/apache/tools/ant/taskdefs/defaults.properties
ant/core/trunk/src/main/org/apache/tools/ant/taskdefs/optional/extension/resolvers/